Polymer microparticles, differently from other polymer molded articles such as films, fibers, injection molded articles or extrusion molded articles, are characterized in that the specific surface area is high and the shape is spherical, and are used in modification or improvement for various materials. As major uses thereof, modifiers for cosmetics, additives for toners, additives for paints, additives to molded articles, light diffusing agents for films and the like can be exemplified.
Ethylene-vinyl alcohol copolymer is excellent in gas barrier property for oxygen and the like, oil resistance, organic solvent resistance, aroma retaining property, weatherability, and transparency. Thus, it is often used as coating materials for raw materials for wrapping materials, plastic molded articles, metals, wooden materials and the like.
In particular, in a wrapping for products of foods, medicines, electronic parts and the like, for the purpose of retaining the qualities and performance of the products by suppressing invasion of oxygen or moisture, materials applied with barrier coating with ethylene-vinyl alcohol copolymer are frequently used.
Generally, as a method of obtaining a coating layer of ethylene-vinyl alcohol copolymer, methods such as melt extrusion, injection molding, or film lamination, are broadly employed.
Layers obtained by those methods are generally thick and, to enlarge utilization of the barrier coating layer of ethylene-vinyl alcohol copolymer, a method capable of coating a thin coating layer to a complicated shape is desired. As such a method, a method of forming a thin coating layer using a solution or a dispersion of ethylene-vinyl alcohol copolymer is known.
In that connection, in a process in which a coating layer is dried and formed after the solution or dispersion is applied, to reduce the volatile amount of an organic solvent, it is preferred to use an aqueous dispersion prepared by dispersing ethylene-vinyl alcohol copolymer with poor water solubility into water.
As the method of obtaining an aqueous dispersion of ethylene-vinyl alcohol copolymer, there is a method of obtaining an aqueous dispersion of ethylene-vinyl alcohol copolymer microparticles by copolymerizing ethylene and vinyl ester monomer in an organic solvent and then taking out ethylene-vinyl alcohol copolymer microparticles by adding water to the system to cause phase inversion emulsification, and saponifying the obtained microparticles (JP HEI-3-250005 A). Further, there is a method of obtaining an aqueous dispersion of microparticles by adding a dispersion stabilizer to a solution of ethylene-vinyl alcohol copolymer, precipitating ethylene-vinyl alcohol copolymer microparticles by cooling the solution, and then carrying out solvent substitution (JP 2001-234019 A).
When an aqueous dispersion of ethylene-vinyl alcohol copolymer is formed as a coating layer, if the particle size distribution of the ethylene-vinyl alcohol copolymer microparticles in water is narrow and the particle surfaces are smooth and uniformly spherical, flowability of the microparticles is improved, workability at the time of forming a coating layer is improved, and it becomes possible to form a coating layer along a complicated shape, and a barrier coating layer obtained by drying the solvent also becomes smooth and uniform.
However, in the method described in JP HEI-3-250005 A, it is difficult to suppress coagulation of the particles during saponification, it is difficult to suppress the particle size distribution of ethylene-vinyl alcohol copolymer microparticles to be obtained to be narrow and, therefore, even if such a dispersion is used in a barrier coating process, it is difficult to obtain a uniformly thin coating layer. Also, in the aqueous dispersion disclosed in JP 2001-234019 A, as the result of detailed investigation with respect to its practicality, the shape of the obtained particles is not a true spherical shape, but an irregular shape. Further, the particles form aggregates and, thus, particles having a narrow particle size distribution have not been obtained.
As the form of ethylene-vinyl alcohol copolymer microparticles, in consideration of the practical use, an aqueous dispersion of ethylene-vinyl alcohol copolymer microparticles is preferred from the viewpoint of easy handling, but in consideration of transportation and storage, a dry-powder state is desirable from the economic viewpoint.
Furthermore, when the dry powder is added again to an aqueous solvent, it is required that the powder exhibits the same dispersibility and particle size distribution also in the liquid with good repeatability. Namely, it is preferred to obtain dry powder of ethylene-vinyl alcohol copolymer microparticles exhibiting a uniform dispersibility without forming aggregates even if being added into an aqueous solvent. However, JP HEI-3-250005 A and JP 2001-234019 A are limited to a preparation method of an aqueous dispersion, and we found that, if the microparticles are once dried, bulky and strong aggregates are formed, even if the dry powder is added again into an aqueous solvent, the microparticles are not re-dispersed, and a uniform dispersion cannot be obtained.
It could therefore be helpful to provide ethylene-vinyl alcohol copolymer microparticles narrow in particle size distribution in a dry-powder state and high in sphericity of particles. Further, it could be helpful to provide ethylene-vinyl alcohol copolymer microparticles with a true spherical shape having a good re-dispersibility, which have not only a narrow particle size distribution at a dry-powder state, but also a high dispersibility without causing coagulation even in a liquid, and have an average particle diameter equal to that in a dry-powder state even in a form of an aqueous dispersion.